Phase change technology is a promising technology for next generation memories. It uses chalcogenide semiconductors for storing states. The chalcogenide semiconductors, also called phase change materials, have a crystalline state and an amorphous state. In the crystalline state, the phase change materials have a low resistivity, while in the amorphous state they have a high resistivity. The resistivity ratios of the phase change materials in the amorphous and crystalline states are typically greater than 1,000 and thus the phase change memory devices are unlikely to have errors for reading states. The chalcogenide materials are stable at certain temperature ranges in both crystalline and amorphous states and can be switched back and forth between the two states by electric pulses. One type of memory device that uses the principal of phase change in chalcogenide semiconductors is commonly referred to as phase change random access memory (PRAM). Phase change memories have the advantageous feature of having small cell sizes, thus can be used for forming high-density memories.
One engineering challenge in improving PRAM devices is to provide enough programming current to effectuate the reversible phase change. Conventionally, MOS devices are used as selectors for the selection and programming of PRAM devices. However, MOS devices typically have relatively small driving currents. The reliability of the programming is thus adversely affected. Bipolar junction transistors (BJT) are thus preferred.
Due to small sizes and high scalability, vertical BJTs are good candidates for being used as selectors. FIG. 1 illustrates a conventional vertical PNP BJT 2, which is formed on substrate 12. PNP BJT 2 includes a p-type collector 6, an n-type base 8 on collector 6, and an emitter 10 on base 8. BJT 2 is encircled by an isolation structure 4, which has a top surface 14 level with or lower than a top surface 16 of isolation structure 4.
One drawback of the vertical BJTs is that with the scaling of integrated circuits, the depth of isolation structure 4 is reduced. For 45 nm technology and beyond, the depth of isolation structure is too small to accommodate all of the emitter 10, base 8 and collector 6. As a result, serious leakage may occur.
Therefore, what is needed in the art is a selector that may incorporate vertical BJTs to take advantage of the benefits associated with the small sizes and high scalability while at the same time overcoming the deficiencies of the prior art.